1. Field of the Invention
The present invention relates to a packaging device which stretches and covers a cover film onto a packaged item accommodated in a mounded fashion in a tray, and welds the film onto the flange section of the tray, in the conveyance path of the tray in a conveyor line.
2. Description of the Related Art
The invention disclosed in U.S. Pat. No. 6,834,476B presented by the same applicants as the present case is a packaging device for covering and sealing a cover film onto a tray, in which a sealing frame is moved back and forth reciprocally above the conveyance path of a band-shaped cover film, and a sealing base performs a box motion operation in conjunction with the reciprocal movement of the sealing frame, a tray together with a packaged item being pushed upwards by the upward movement in the box motion operation of the sealing base, the film thus being stretched by the upward pushing of the packaged item, and the film becoming sealed to the flange section of the tray by being pressed against the upper-positioned sealing frame. According to this device, the film is pushed and stretched from below by an upward impact from the packaged item, and the film can be welded to the flange section of the tray.
The chain conveyor conveys respective trays in which packaged items are mounded, at equidistant intervals, in a horizontal direction, and the sealing frame moves back and forth reciprocally above the trays in synchronism with the conveyance pitch of the trays. On the other hand, the sealing base performs a box motion operation in conjunction with the reciprocal movement of the sealing frame.
However, since this device causes the tray to press against an upper-positioned sealing frame, by means of a box motion operation of the lower sealing base only, the amount of vertical movement of the sealing base is large, and the bottom face of the tray becomes separated from the receiving plate which supports it. In this separated state, the tension in the film acts directly downwards on the tray, through the packaged item accommodated in a mounded fashion in the tray, and therefore, depending on the circumstances, the flange of the tray may break, or the flange may be bent backwards due to the load, thus presenting an obstacle to the welding between the flange and the cover film.